f test - Two Sample in Excel using the QI Macros

f test - two sample

In the constant quest to reduce variation and improve products, companies need to evaluate different alternatives. A f test using two samples compares two independent sets of test data. It helps determine if the variances are the same or different from each other. Consider the following example.

f test two sample example

Watch the f test in Excel Video

If you're producing rubber made with two different recipes, you might want to know if the variances in tensile strengths are the same or different (Juran's QC Handbook 4th pg 23.74):

• The null hypotheis (H₀₎ - variances are the same
• The alternate hypothesis (H_a) - variances are different

Now, conduct a test and enter the data into Excel:

Select the data with the mouse and click on the QI Macros Menu to select the F test:

The QI Macros will prompt for a significance level (default = 0.05):

The f test macro will calculate the results:

Interpreting the F test results

Since F < Fcrit (.33 < 6.39) and p value > a ( .152 > 0.05), we can accept the null hypothesis that the variances are equal.

This may be a useful first step before running a t test.

Excel Note:
If you run the test using, Excel's Data Analysis toolpak, Excel requires that the recipe with the largest variance be first to ensure correct calculations. The QI Macros does not have this requirement to perform the calculations correctly.

Watch the f test in Excel Video

The f test is just one of the tests included in the QI Macros Statistical Process Control software. Other tests include:

• Levenes Test of variance for non-normal data
• t tests
• z test two sample for means
• ANOVA - Analysis of Variance
• Overvew of all hypothesis tests in Excel

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